Method and composition for the desulfurization of molten metals

ABSTRACT

Molten metals, especially molten pig iron, are desulfurized by contacting them with a composition comprising calcium carbide or calcium cyanamide and an additive agent yielding water or hydrogen at the temperature of the molten metal; preferred as the additive agents are the alkali metal hydrides, polyethylene or polyamide for yielding hydrogen and hydrate of lime and alkaline earth borates for yielding water.

The present invention relates to compositions for the desulfurization of molten metals, especially of molten pig iron. The compositions contain calcium carbide and/or calcium cyanamide with an additive increasing the desulfurization effect.

Sulfur contained in pig iron impairs especially the mechanical properties of ferrous materials and therefore appreciable amounts of it in such materials are undesirable. Since the selection and procurement of starting materials for the manufacture of low-sulfur ferrous products is becoming increasingly difficult, the molten irons in general must be subjected to a desulfurizing treatment. However, in the production of steel from pig iron the removal of the sulfur from the molten iron is difficult and uneconomical; it is more advantageous to initially reduce the sulfur in the pig iron to sufficiently low levels.

Known methods of desulfurizing molten iron outside of the melting unit make use of desulfurizing agents consisting of two or more solids in fine powdered form. These are fluidized by means of a carrier current of gas--air, nitrogen, argon, natural gas and other neutral gases or gases having a reducing action may be used--and blown into the molten iron. The reaction between the solid desulfurizing agent and the sulfur bound to the iron takes place on the surface of the desulfurizing agent.

Also known are desulfurization processes in which calcium cyanamide or calcium carbide are blown into the molten iron together with fine powdered carbon materials such as soft coal, anthracite, brown coal, coke, petroleum coke and other products containing carbon, which provide a reducing atmosphere conducive to desulfurization.

A definite advance has been achieved by desulfurization with combinations of calcium cyanamide or calcium carbide and diamide lime (W. German Pat. Nos. 1,583,268 and 1,758,250). Such agents not only create within the molten metal the desired reducing atmosphere in which the desulfurizing agent produces its effect without delay, but also, by the simultaneous yielding of gas from the diamide lime, they promote the uniform distribution of the desulfurizing agent into all parts of the melt and accelerate the precipitation of the desulfurization products.

In spite of these good results, there has been a need in metallurgical plant practice to improve desulfurizing agents based on calcium cyanamide and/or calcium carbide with regard to the degree of desulfurization which they achieve and with regard to their accuracy and reliability in achieving low sulfur content levels.

The present invention provides a desulfurization composition capable of achieving these objectives.

Essentially, the invention comprises a desulfurizing composition based on calcium carbide and/or calcium cyanamide and containing an agent which yields hydrogen and/or water at the temperature of the molten metal being treated with the solid desulfurizing compositions.

Suitable agents are, for example:

a. For yielding H₂ : calcium hydride and the hydrides of other alkaline earth and alkali metals, organic polymers containing hydrogen, e.g., polyolefins such as polyethylene and polypropylene, polyamides, polystyrene, and polyacrylonitrile, either individually or in mixtures, as well as urea, guanidines, biguanidines, dicyandiamide, dicyandiamidine and melamine.

b. For yielding H₂ O: calcium hydroxide (hydrate of lime, Ca(OH)₂), alkaline earth borates containing water of crystallization, such as colemanite and pandermite, aluminum hydroxides, perlite, kaolin, clays and other such minerals, carbohydrates such as sugar and starch, solid organic oxygen compounds such as phthalic acid and glycolic acid, organic polymers containing hydrogen and oxygen such as polyvinyl alcohol and polyvinyl acetate, and polyalcohols such as sorbitol.

The organic polymers may be prepared by many different polymerization processes and in many different degress of polymerization. The nitrogen simultaneously yielded by nitrogen-containing additives during the treatment does not impair the desulfurization effect.

Hydrate of lime is preferred as the H₂ O yielding agent, since it is available at low cost virtually anywhere in the world without high transportation cost.

The decomposition of the powdered agent of the invention forms a desirable reducing atmosphere even before the actual desulfurizing agent begins its action. The agents decompose spontaneously at the temperatures of the molten metal (from about 1200 to 1450° C. in the case of iron) with the formation of water or hydrogen, nitrogen in some cases, and in some cases very finely divided carbon. The carbon, in the active form in which it is thus produced, exercises an advantageous action partially by binding the small amounts of oxygen dissolved in the iron, but mainly by forming carbon monoxide with the oxygen content of the desulfurizing agent or reacting with the oxygen in the carrier gas, or by forming carbon dioxide from carbonate components. The gases that are produced intensify the turbulence in the melt, increase the movement of the bath and assure the reducing status.

It has been found desirable for the amount of the agents to range from 0.3 to 60% by weight, the amount of hydrogen gas yielding substances being best between 0.3 and 20%, the amount of water yielding substances between 1 and 60%, preferably 5 and 40%; in the case of carbohydrates 1 to 30% will suffice.

According to a special embodiment of the invention, the desulfurizing composition of the invention will additionally contain deoxidizers such as aluminum or calcium silicon in amounts of up to about 10 % by weight or carbon in amounts of up to about 20 % by weight. In this manner, the desired reducing atmosphere is favored. The basic desulfurizing agent, calcium carbide or calcium cyanamide, is present in an amount of at least 30%, preferably at least 45%, by weight.

Especially advantageous mixtures have the following composition as shown in the indicated tables below showing performance data.

    ______________________________________                                         1)  Calcium carbide                                                                              60-90%                                                           Diamide lime  5-39,7%       (cf. Table HT 10, 11) - Polyethylene 0.3-5                                     %                                              2)  Calcium carbide                                                                              85-99%                                                                                       (cf. Table HT 8)                                   Dicyandiamide 1-15%                                                        3)  Calcium carbide                                                                              60-80%,       especially 72-78%                                  Carbon        5-20%,        especially 5-7% *                                  Ca(OH).sub.2  5-35%,        especially 15-23%                              4)  Calcium cyanamide                                                                            60-85%                                                           Carbon        1-10%         (cf. Table HT 19)                                  Ca(OH).sub.2  5-30%                                                        5   Calcium cyanamide                                                                            60-80%                                                           Diamide lime  18-39,7%      (cf. Table HT 12)                                  Polyethylene  0.3-2%                                                       6)  Calcium carbide                                                                              65-95%                                                                                       (cf. Table B 14)                                   Ca(OH).sub.2  5-35%                                                        7)  Calcium carbide                                                                              90-99,5%                                                                                     (cf. Table HT 7)                                   Polyethylene  0.5-10%                                                      8)  Calcium carbide                                                                              60-98%                                                           Alkaline earth                                                                               2-40%         (cf. Table B 15)                                    borate                                                                    9)  Calcium cyanamide                                                                            85-99%                                                                                       (cf. Table HT 9)                                   Dicyandiamide 1-15%                                                        10) Calcium hydroxide                                                                            40-95%                                                           Diamide lime  0-49,7%       (cf. Table HT 27)                                  Polyethylene  0,3-20%                                                      11) Calcium carbide                                                                              30-99,7%                                                         Calcium hydroxide                                                                            0-50%         (cf. Table B 29)                                   Dicyandiamide 0.3-20%                                                      12) Calcium carbide                                                                              30-95%                                                           Diamide lime  0-49,7%       (cf. Table B 26)                                   Dicyandiamide 0.3-20%                                                      13) Calcium carbide                                                                              30-95%                                                           Calcium hydroxide                                                                            1-60%         (cf. Table HT 28)                                  Polyethylene  0.3-10%                                                      14) Calcium carbide                                                                              30-95%                                                           Carbon        0-20%         (cf. Table B 30)                                   Calcium hydroxide                                                                            5-60%                                                        15) Calcium carbide                                                                              30-95%                                                           Carbon        0-20%         (cf. Table HT 31)                                  Colemanite    5-50%                                                        16) Calcium carbide                                                                              50-80%                                                           Diamide lime  10-20%                                                                                       (cf. Table B 32)                                   Coke dust     1-15%                                                            Colemanite    5-15%                                                        17) Calcium carbide                                                                              50-80%                                                           Coke dust     5-20%         (cf. Table B 33)                                   Colemanite    10-30%                                                       18) Calcium cyanamide                                                                            75-95%                                                           Alkaline earth                                                                               5-25%         (cf. Table HT 16)                                   borate                                                                    19) Calcium carbide                                                                              30-90%                                                           Diamide lime  0-49%                                                                                        (cf. Table B 22)                                   Alkaline earth                                                                               1-40%                                                             borate                                                                    20) Calcium carbide                                                                              60-80%                                                           Petroleum coke                                                                               15-30%        (cf. Table HT 20)                                  Polyvinyl alcohol                                                                            5-10%                                                        ______________________________________                                          * (cf. Table HT 17, 18)                                                  

All percentages given refer to the weight, unless otherwise specified.

The desulfurizing agents of the invention are prepared by mixing the components, whereupon moisture adhering to the agent reacts with the basic desulfurizing agent with the formation of acetylene (in the case of CaC₂) or Ca(OH)₂, so as to assure that the agent can contain only bound H₂ O.

The desulfurizing agents of the invention provide additional effects when they are used, so that the amount of desulfurizing agent used is less than it has been in the case of the agents known hitherto, or the degree of desulfurization is greater for the same amount. Final sulfur contents are attained of 0.02 % S_(E) to 0.01 % S_(E) for a starting sulfur content of 0.04 to 0.15 % S_(A), with the accuracy desired in modern-day practice.

With the mixtures of the invention equally good results are achieved in the desulfurization of molten pig iron and ferrous alloys such as ferrochromium and ferronickel, and also in nonferrous molten metals such as nickel, copper and the like.

The invention will be explained with the aid of the following examples. Examples 1-6 contain comparisons with desulfurizing agents of the prior art, and Examples 7-24 show the effect of the desulfurizing agent of the invention.

    __________________________________________________________________________     Examples for Purposes of Comparison:                                                 Base              Identical to                                           No.   Composition                                                                              Agent   equivalent amounts                                     __________________________________________________________________________     B 1   Calcium carbide                                                                          --      --                                                     B 2   Calcium cyanamide                                                                        --      --                                                     B 3   Calcium carbide                                                                + carbon  --      --                                                     B 4   Calcium cyanamide                                                              + carbon  --      --                                                     B 5   Calcium carbide                                                                + diamide lime                                                                           --      --                                                     B 6   Calcium cyanamide                                                              + diamide lime                                                                           --      --                                                     Agents which evolve H.sub.2 :                                                  HT 7  Calcium carbide                                                                          Polyethylene                                                                           Polypropylene                                                                  instead of poly-                                                               ethylene                                               HT 8  Calcium carbide                                                                          Dicyandiamide                                                                          Dicyandiamidine,                                                               melamine, urea,                                                                polyacryloni-                                                                  trile, instead                                                                 of dicyandiamide                                       HT 10,11                                                                             Calcium carbide                                                                + diamide lime                                                                           Polyethylene                                                   HT 13 Calcium carbide                                                                + diamide lime                                                                           Polyamide                                                                              Calcium cyanamide                                                              instead of                                                                     calcium carbide                                                                + diamide lime                                         HT 9  Calcium cyanamide                                                                        Dicyandiamide                                                                          Identical to                                                                   polyethylene or                                                                polyamide                                                                      instead of                                                                     dicyandiamide                                          HT 12 Calcium cyanamide                                                                        Polyethylene                                                                           Calcium cyanamide                                            + diamide lime    without diamide                                                                lime                                                   HT 26 Calcium carbide                                                                + diamide lime                                                                           Dicyandiamide                                                  HT 27 Calcium carbide                                                                + diamide lime                                                                           Polyethylene                                                   Agents which evolve H.sub.2 O:                                                 B 14  Calcium carbide                                                                          Calcium                                                                        hydroxide                                                      B 15  Calcium carbide                                                                          Alkaline earth                                                                 borate                                                         HT 17 Calcium carbide                                                                          Calcium Aluminum hydro-                                        & 18  + carbon  hydroxide                                                                              xide instead of                                                                calcium hydroxide                                      HT 20 Calcium carbide                                                                          Polyvinyl                                                                              Starch, sorbitol,                                            + carbon  alcohol polyvinyl acetate                                                              and other organic                                                              oxygen compounds                                                               instead of                                                                     polyvinyl alcohol.                                     HT 21 Calcium carbide                                                                          Perlite Kaolin, clay                                                 + diamide lime                                                           B 22  Calcium carbide                                                                          Alkaline earth                                                       + diamide lime                                                                           borate                                                         HT 24 Calcium carbide                                                                          Alkaline earth                                                       + diamide lime                                                                           borate +                                                                       aluminum                                                       HT 16 Calcium cyanamide                                                                        Alkaline earth                                                                 borate                                                         HT 19 Calcium cyanamide                                                                        Calcium Cane sugar instead                                           + carbon  hydroxide                                                                              of calcium                                                                     hydroxide                                              HT 23 Calcium cyanamide                                                                        Alkaline                                                             + diamide lime                                                                           earth                                                                          borate                                                         B 25  Calcium carbide                                                                          Calcium                                                                        hydroxide                                                      B 30  Calcium carbide                                                                          Calcium                                                              + carbon  hydroxide                                                      HT 31 Calcium carbide                                                                          Colemanite                                                           + carbon                                                                 B 32  Calcium carbide                                                                          Colemanite                                                           + diamide lime                                                                 + carbon                                                                 B 33  Calcium carbide                                                                          Colemanite                                                           + carbon                                                                 Agents which evolve H.sub.2 and H.sub.2 O:                                     HT 28 Calcium carbide                                                                          Calcium                                                                        hydroxide                                                                      Polyethylene                                                   B 29  Calcium carbide                                                                          Calcium                                                                        hydroxide                                                                      Dicyandiamide                                                  __________________________________________________________________________

The rest of the agents named are also usable in the same manner. Which agent is actually used will vary locally according to economic criteria.

EXAMPLES 1 to 24

The results given in the following table are averages obtained from up to 6 desulfurization tests where experiments on a pilot plant scale (HT) are involved. Where the results are based on factory tests (B) the desulfurization was performed in torpedo ladles containing approximately 200 metric tons of pig iron, based on an average of more than 20 treatments.

In all experiments, the powdered desulfurization agents were blown into a pig iron melt through refractory-jacketed blowing lances using air as the carrier-gas.

The α-value given in the table is a characteristic which expresses the consumption of desulfurization agent in kilograms per metric ton of pig iron and a decrease of 0.01 % in the sulfur content of the pig iron.

    ______________________________________                                         Initial sulfur content                                                                             = S.sub.A                                                  Final sulfur content                                                                               = S.sub.E                                                  Degree of desulfurization.sup.+                                                                     ##STR1##                                                  Difference between S.sub.A and S.sub.E                                                             = Δ.sub.S                                            ______________________________________                                          .sup.+ "E"-Rating                                                        

    __________________________________________________________________________     Experiment                                                                           Base          Type of        "E"                                          No.  Composition                                                                              %   Agent      kg/t                                                                               Rating                                                                             α                                                                            S.sub.A                                                                             S.sub.E                                                                             ΔS                  __________________________________________________________________________     B 1   Calcium carbide                                                                          100 --         5.2 60  1.80                                                                               0.048                                                                               0.019                                                                               29                        B 2   Calcium cyanamide                                                                        100 --         14.2                                                                               65  2.80                                                                               0.085                                                                               0.035                                                                               50                        B 3   Calcium carbide                                                                          70  --         3.75                                                                               66  1.50                                                                               0.038                                                                               0.013                                                                               25                              Petroleum coke                                                                           30                                                             B 4   Calcium cyanamide                                                                        95  --         8.0 69  1.95                                                                               0.060                                                                               0.019                                                                               41                              Coke dust 5                                                              B 5   Calcium carbide                                                                          75  --         4.2 55  1.31                                                                               0.058                                                                               0.026                                                                               32                              diamide lime                                                                             25                                                             B 6   Calcium cyanamide                                                                        70  --         8.5 60  2.18                                                                               0.065                                                                               0.026                                                                               39                              diamide lime                                                                             30                                                             HT 7  Calcium carbide                                                                          94  + 6% Poly- 5.2 75  1.45                                                                               0.048                                                                               0.012                                                                               36                                            ethylene                                                   HT 8  Calcium carbide                                                                          93  + 7% Dicyan-                                                                              4.4 69  1.42                                                                               0.045                                                                               0.014                                                                               31                                            diamide                                                    HT 9  Calcium cyanamide                                                                        92.5                                                                               + 7,5% Dicyan-                                                                            7.2 64  1.84                                                                               0.061                                                                               0.022                                                                               39                                            diamide                                                    HT 10 Calcium carbide                                                                          82.5                                                                               + 2,5% Poly-                                                                              6.0 83  1.25                                                                               0.058                                                                               0.010                                                                               48                              diamide lime                                                                             15  ethylene                                                   HT 11 Calcium carbide                                                                          74.7                                                                               +0.3% Poly-                                                                               5.5 80  1.41                                                                               0.049                                                                               0.010                                                                               39                              diamide lime                                                                             25  ethylene                                                   HT 12 Calcium cyanamide                                                                        69.7                                                                               +0.3% Poly-                                                                               10.5                                                                               69  1.95                                                                               0.078                                                                               0.024                                                                               54                              diamide lime                                                                             30  ethylene                                                   HT 13 Calcium carbide                                                                          70  +2.5% Poly-                                                                               5.5 70  1.37                                                                               0.057                                                                               0.017                                                                               40                              diamide lime                                                                             27.5                                                                               amide                                                      B 14  Calcium carbide                                                                          80  +20%       5.4 76  1.45                                                                               0.049                                                                               0.012                                                                               37                                            Ca(OH).sub.2                                               B 15  Calcium carbide                                                                          85  +15%       5.8 70  1.41                                                                               0.059                                                                               0.018                                                                               41                                            Colemanite                                                 HT 16 Calcium cyanamide                                                                        80  +20%       6.5 67  1.55                                                                               0.063                                                                               0.021                                                                               42                                            Colemanite                                                 HT 17 Calcium carbide                                                                          70  +20%       5.5 72  1.37                                                                               0.056                                                                               0.016                                                                               40                              Coke dust 10  Ca(OH).sub.2                                               HT 18 Calcium carbide                                                                          60  +30%       5.2 67  1.40                                                                               0.055                                                                               0.018                                                                               37                              Coke dust 10  Ca(OH).sub.2                                               HT 19 Calcium cyanamide                                                                        75  +20%       7.5 68  1.78                                                                               0.062                                                                               0.020                                                                               42                              Coke dust 5   Ca(OH).sub.2                                               HT 20 Calcium carbide                                                                          70  + 10% Poly-                                                                               5.8 80  1.42                                                                               0.051                                                                               0.010                                                                               41                              Petroleum coke                                                                           20  vinyl alcohol                                              HT 21 Calcium carbide                                                                          60  + 30% Perlite                                                                             5.3 89  1.29                                                                               0.051                                                                               0.010                                                                               41                              diamide lime                                                                             10                                                             B 22  Calcium carbide                                                                          65  + 10% Cole-                                                                               5.0 80  1.19                                                                               0.053                                                                               0.011                                                                               42                              diamide lime                                                                             25  manite                                                     HT 23 Calcium cyanamide                                                                        63  + 17% Pander-                                                                             6.5 72  1.38                                                                               0.065                                                                               0.018                                                                               47                              diamide lime                                                                             20  mite                                                       HT 24 Calcium carbide                                                                          60  + 15% Cole-                                                                               4.8 82  1.14                                                                               0.051                                                                               0.009                                                                               42                              diamide lime                                                                             20  manite                                                           Aluminum  5                                                              B 25  Calcium carbide                                                                          65  + 35% Calcium                                                                             5.8 76  1.42                                                                               0.061                                                                               0.020                                                                               41                                            hydroxide                                                  B 26  Calcium carbide                                                                          65  + 5% Dicyan-                                                                              5.4 73  1.26                                                                               0.059                                                                               0.016                                                                               43                              diamide lime                                                                             30  diamide                                                    HT 27 Calcium carbide                                                                          60  + 6% Poly- 3.9 68  1.30                                                                               0.044                                                                               0.014                                                                               30                              diamide lime                                                                             34  ethylene                                                   HT 28 Calcium carbide                                                                          60  35% Ca(OH).sub.2                                                                          4.6 62  1.39                                                                               0.053                                                                               0.020                                                                               33                                             5% Polyethylene                                           B 29  Calcium carbide                                                                          60  34% Ca(OH).sub.2                                                                          5.4 70  1.28                                                                               0.060                                                                               0.018                                                                               42                                             6% Dicyandiamide                                          B 30  Calcium carbide                                                                          45  + 40% Calcium                                                                             4.6 63  1.44                                                                               0.051                                                                               0.019                                                                               32                              Carbon    15  hydroxide                                                  HT 31 Calcium carbide                                                                          50  + 35% Cole-                                                                               6.3 73  1.40                                                                               0.062                                                                               0.017                                                                               45                              Carbon    15  manite                                                     B 32  Calcium carbide                                                                          75                                                                   diamide lime                                                                             12.5                                                                               + 9% Cole- 7.0 77  1.37                                                                               0.066                                                                               0.015                                                                               51                              Coke dust 3.5 manite                                                     B 33  Calcium carbide                                                                          75  + 18% Cole-                                                                               7.0 80  1.32                                                                               0.066                                                                               0.013                                                                               53                              Coke dust 7   manite                                                     __________________________________________________________________________

It will be understood that the foregoing specification and examples are illustrative but not limitative of the present invention inasmuch as other embodiments within the spirit and scope of the invention will suggest themselves to those skilled in the art. 

What is claimed is:
 1. Solid composition for the desulfurization of molten metals consisting essentially of at least one of calcium carbide and calcium cyanamide as the desulfurizing agent present in an amount of at least 30% by weight, and, in addition, an additive agent which yields at least one of hydrogen and water at the temperature of the molten metal being treated thereby forming a reducing atmosphere for the desulfurization.
 2. Composition as claimed in claim 1, wherein there is additionally present a deoxidizing substance.
 3. Composition as claimed in claim 2, wherein said deoxidizing substance is at least one of carbon, aluminum or calcium-silicon.
 4. Composition as claimed in claim 1, wherein said additive agent is a hydrogen-yielding solid substance present in an amount of from 0.3 to 20% by weight of the total composition.
 5. Composition as claimed in claim 4, wherein said hydrogen-yielding solid substance is selected from the group of calcium hydride, a polyolefin, a polyamide, a polystyrene, a polyacrylonitrile, urea, guanidine, biguanidine, dicyandiamide, dicyandiamidine, and melamine.
 6. Composition as claimed in claim 5, wherein said polyolefin is polyethylene or polypropylene.
 7. Composition as claimed in claim 1, wherein said additive agent is calcium hydroxide contained in an amount of from 1 to 60% by weight of the total composition.
 8. Composition as claimed in claim 7, wherein said amount is from 5 to 40% by weight.
 9. Composition as claimed in claim 1, wherein said additive agent is a carbohydrate contained in an amount of from 1 to 30% by weight of the total composition.
 10. Composition as claimed in claim 1, wherein said additive agent is an alkaline earth borate containing water of crystallization contained in an amount of from 1 to 50% by weight of the total composition.
 11. Composition as claimed in claim 1 consisting essentially of the following:Calcium cyanamide; 60 -- 80% Diamide lime; 18 -- 39.7% Polyethylene; 0.3 -- 2% by weight of total composition.
 12. Composition as claimed in claim 1 consisting essentially of the following:Calcium carbide; 40 -- 95% Diamide lime; 0 -- 49.7% Polyethylene; 0.3 -- 10% by weight of total composition.
 13. Composition as claimed in claim 1 consisting essentially of the following:Calcium carbide; 60 -- 90% Diamide lime; 5 -- 39.7% polyethylene; 0.3 -- 5% by weight of total composition.
 14. Composition as claimed in claim 1 consisting essentially of the following:Calcium carbide; 30 -- 95% Calcium hydroxide; 1 -- 60% Polyethylene; 0.3 -- 10% by weight of total composition.
 15. Method of desulfurizing a molten metal, which method comprises contacting said metal, at a temperature of from about 1200° to 1450° C., with a composition as claimed in claim
 1. 16. Method as claimed in claim 15, wherein said metal is molten pig iron.
 17. A process for desulfurizing molten pig iron comprising introducing a mixture of calcium carbide and a solid material selected from the group consisting of polyethylene, in a carrier gas, to the molten pig iron.
 18. The process of claim 17 wherein a substance which generates carbon dioxide selected from the group consisting of limestone is added to the mixture. 